The present invention relates to an apparatus and method for mounting printing plates and proofing and, more particularly, to a an apparatus having an optical alignment system incorporating video cameras. The apparatus is applicable in mounting plates having microdots for registration marks and other plate mounting situations.
In multicolor printing, printing cylinders or sleeves have printing plates mounted thereon corresponding to a desired color to be printed. Each printing plate for a given image color must be in correct relative alignment with the other color plates for the image which are mounted on other printing cylinders or sleeves for the plates to align on the same portion of web material during printing. Conventional mounting and proofing techniques for multicolor printing have employed registration targets on printing plates to allow proper alignment of printing plates on a either a printing cylinder or plate sleeve to be installed on a printing cylinder. Such targets generally consist of a circle of approximately quarter-inch diameter with cross-hairs therein. The targets are located on the printing plate at centers of each side and outside of the pattern desired to be printed. Thus, the final product has the desired printed image appearing without the targets being visible because the targets are printed on a portion of material which is either covered or removed. For example, in the converting industry which processes corrugated cardboard and other materials in to packaging products, such targets are typically located on flaps of boxes folded under opposing flaps or on bottoms and therefore are out of sight.
The use of registration targets is effective and provides for satisfactory alignment of plates. However, since the registration targets are readily visible, it is standard practice to put them outside of image sights. This requires that the plates be made larger than necessary for printing a given image. Additionally, since the printing cylinder or sleeve will generally have several plates mounted proximate to each other, the increased size of the plates can restrict flexibility in adjacent plate mounting.
Another system of registration includes pin registration. The plates are drilled at precise locations and the holes are then lined up with pins on the printing cylinder. This type of registration is traditionally used in offset printing where tight tolerances are maintained. However, flexographic printing has now adopted this method as the stability of flexible printing plates has improved due to advances in polymer technology. Tolerances of 1/32 of an inch are considered standard. While accurate, the registration technique is time consuming because extra operations are required to drill the printing plates using photonegatives as guides.
Conventional mounting techniques and machinery are exemplified by optical mounting and proofing devices disclosed in U.S. Pat. No. 2,289,557, A. K. Taylor; U.S. Pat. No. 2,452,373, E. L. Harley; U.S. Pat. No. 2,561,115, E. L. Harley; U.S. Pat. No. 2,492,798, E. L. Harley; and U.S. Pat. No. 2,493,628, E. L. Harley. Some of the disclosed devices are directed to flat bed printing while other are directed to cylinder printing. Regardless of the particular printing medium, the principle of operation remains generally consistent and therefore is discussed with reference to cylinder printing. Plates are positioned on a plate cylinder (also known as a form cylinder) while being optically compared with a layout sheet (sometimes called a registration sheet) on a layout cylinder (also called a mounting or proofing cylinder), or corresponding plates on a second plate cylinder. Optical systems of the devices use mirrors to superimpose images of the plates being mounted with corresponding images, or registration marks, on either the layout cylinder or a second plate cylinder. When the operator has the plates accurately aligned, the plates are held in place on the plate cylinder (which may in fact be a printing cylinder or a cylinder carrying a sleeve to be mounted on a printing cylinder) with stickyback material, a form of double-sided adhesive tape. Once the plates are secured in place, a proof may optionally be printed on the layout cylinder to ensure proper correspondence. Alternatively, because of the simplicity and accuracy of the visual alignment of the plates, time savings can be achieved by eliminating the proofing step. An example of one such machine is the Opti-Chek.RTM. Mounting and Proofing Machine Series 4000 produced by E. L. Harley Inc. of Brooklyn, N.Y.
Alignment of the plates using the above devices may be achieved by either aligning images themselves or target type registration marks. The layout cylinder has a layout sheet mounted thereon whereupon outlines and/or targets have been drawn to permit alignment of the plates. The plates are aligned with the layout so that proper alignment with the web and the other plates is achieved. Sometimes plates will have openings for the incorporation of smaller plates called "slugs." The use of slugs permits the individualizing of print runs. A standard product may be package by several distributors, each of which uses packaging material with the same basic layout but requires that the individual packages be identified somewhere on the packaging. Thus, slugs are added to a standard plate mounting to include such specialized information without the expense of requiring separate printing cylinders or sleeves for each packages. Such smaller plates will usually not include registration target and are aligned by eye with the rest of the standard plate. The optical mirror systems permit the operator to view the larger standard plate against the layout and the slug against any corresponding position marks on the layout sheet.
Another type of registration system called "microdot" is used in offset printing and is now being adopted in flexographic printing. The printing plates are provided with small dots, having a diameter in the range of 0.005 to 0.050 inches, and preferably in the range of 0.005 to 0.010 inches, which replace the conventional registration targets. Instead of being located outside the image sight as is done with registration targets, the microdots are located within the image sight since they are not readily apparent to the untrained eye. The microdots are formed photographically when the flexographic plates are developed are thus have a fixed relationship to the plate image. Since the microdots are incorporated in the image, the size of the plate may be reduced and greater flexibility in the alignment of adjacent plates is achieved. Due to the small size of the microdots, alignment using conventional optical mounting systems discussed above is virtually impossible because the user cannot visually discern the microdots to effect alignment of the microdots with their corresponding marks on the layout sheet of the layout cylinder or corresponding microdots on plates of the second plate cylinder. Furthermore, precise alignment is limited by parallax error introduced by the positioning of the viewers eye relative to the optical mechanism and the layout and plate cylinders.
Mounting systems using video technology are disclosed in U.S. Pat. No. 4,520,389, M. Hornschuh; U.S. Pat. No. 5,132,911, Leader, Jr. et al.; and U.S. Pat. No. 5,031,334, Takamura. In U.S. Pat. No. 4,520,389 a mounting and proofing machine is provided with a layout cylinder and a plate cylinder geared to rotate in unison. First and second video plate cameras are mounted to view the plate cylinder while first and second video layout cameras are mounted to view corresponding locations on the layout sheet mounted on the layout cylinder. Video signals from the cameras are electronically mixed to superimpose images of the layout cylinder over positionally corresponding images of the plate cylinder. By viewing the localized areas of two registration targets on the layout cylinder, the operator positions the printing plate on the plate cylinder with corresponding registration targets on the printing plate superimposed over the registration targets of the layout cylinder. The mounting and proofing machine requires that, in addition to the plate and layout cylinder being properly synchronized, each pair of cameras, first plate and layout cylinder cameras and second plate and layout cylinder cameras must be synchronized in position and movement. Furthermore, the multiple camera construction of the machine requires video mixing electronics for superimposing the video images and maintenance of the proper calibration of the mixing alignment.
The mounting apparatus of U.S. Pat. No. 5,132,911 has a single video camera orthogonally displaceable above an orthogonally displaceable turntable upon which an operator places a printing plate. A plate cylinder, adjacent the turntable, is automatically positioned at a home position in known alignment with the turntable and video camera. A computer then scans the printing plate with the video camera to locate the first and second registration marks and stores their position. Based on the known positions of the registration marks, the turntable is repositioned to place the registration marks in proper alignment with the plate cylinder. Once in proper alignment, the printing plate is moved off the turntable by a pressure roller and onto the plate cylinder while the plate cylinder rotates as required to accept the printing plate at the correct position. By implication, the proper location of the registration marks on the printing plate on the plate cylinder must be calculated and preprogrammed for the apparatus to determine where to position the printing plate on the plate cylinder, especially where multiple plates are to be mounted in relative alignment. Thus, a skilled technician must be able to properly calculate positioning of printing plates or the reference marks on the plate cylinder and program the machine accordingly. Furthermore, since there is no optical comparison with a layout cylinder, the operator has no visual indication of the proper positioning of printing plates or slugs on the plate cylinder with respect to a layout sheet. Likewise, aside from actual proofing, the is no visual indication of proper plate alignment from plate cylinder to plate cylinder to ensure coherence of color alignment.
In U.S. Pat. No. 4,520,389 the mounting device has a video camera mounted below a flat plate holder upon which a printing plate is to be mounted. The camera is rotated to align cross-hairs of the camera with a grid on the flat plate holder. Interposed between the video camera and the flat plate holder is a transparent printing plate carrier which is alignable in X, Y, and .THETA. directions. The printing plate has a corresponding grid countersunk on its face and is place face down in the plate carrier. The plate carrier is then positioned to place the grid of the printing plate into alignment with the camera cross-hairs which have previously been aligned with the grid of the plate holder. The plate holder is then lowered on the printing plate which is attached using double-sided tape. Once again, there is no opportunity for the operator to visually proof the plate image onto a layout sheet in order to ensure alignment. Furthermore, there is no apparent means for multiple plates to be mounted and it is required that the flat plate holder have a standard grid. Finally, the mounting machine does not incorporate a means for producing an actual proof print to test alignment.
In order to provide flexibility in mounted printing plates, and in particular flexographic printing plates, a mounting apparatus is needed which provides for using microdot registration while allowing for direct visual verification of alignment with a layout sheet and the placement of multiple printing plates, including slugs, onto a plate cylinder. Simultaneously, it is desirable that such an apparatus minimize mechanical and electronic complexity in order to provide reliable yet economical operation requiring a minimum of calibration and maintenance while enhancing accuracy of mounting over that of the prior art.